Polytetrafluoroethylene (PTFE or Teflon.RTM.) filters have long been used for a variety of applications. For example, porous filters prepared from PTFE fibers which are laid-down with fluorocarbon polymer binders and then cured, find use in filtration of hot, acidic or caustic fluids. Due to production techniques, staple untreated PTFE fibers of the type used in filters typically are dark brown color because of carbonaceous residue. When incorporated into filtration structures, fibers containing such residues not only are aesthetically undesirable but also present a source for potential contamination of the filtrate.
The following definitions are provided to facilitate an understanding of the invention.
"BIeaching" means reduction of extractable contaminants as well as whitening. Bleaching refers to processes of removing the carbonaceous residues from the fibers and from the filter structure following laydown which may typically be present in amounts as high as about 3 weight percent of the staple fibers and about 2 weight percent of the total weight of a chemically bonded filter media (fluorocarbon binder and unbleached fibers).
"PTFE" means polytetrafluoroethylene.
"FEP" means fluorinated ethylene propylene copolymer.
"PFA" means perfluoroalkoxy polymer.
"Set" or "cure" refers to the binder taking on the desired configuration on the fibers to provide the desired structural integrity.
"Fugitive" refers to a dispersion solution composition which is volatilized, decomposed, and/or removed substantially completely prior to or during drying and curing so as to be essentially absent from a finished structure and not increase the level of extractables in the finished fibrous structure.
With reference to the background art, Hurley et al (Pall Corporation), in U.S. Pat. No. 4,716,074 (hereinafter Hurley et al.), describe a process for making a filter incorporating fibers with a reduced level of contaminating residues. Hurley et al, sets out manufacturing procedures for PTFE fiber-based filters which involve the following steps: 1) selecting the PTFE fibers; 2) selecting a fluorocarbon binder; 3) bleaching and pre-shrinking the fibers; 4) forming a stabilized dispersion of the fibers and a fugitive binder; 5) laying down the dispersion; 6) drying the dispersion; 7) applying the fluorocarbon binder to the laydown; 8) drying the laydown; 9) curing the binder coated laydown; and, finally, 10) treating the laydown to remove extractables.
Suitable polytetrafluoroethylene (PTFE) fibers described in Hurley et al. typically have diameters ranging from about 5 to about 50 micrometers, preferably from about 15 to about 35 micrometers, and have lengths ranging from about 1,000 to about 20,000 micrometers, preferably from about 4,500 to about 8,500 micrometers. Fibers outside those ranges can be used, but with less success. For example, fibers having diameters greater than about 50 micrometers and longer than about 20,000 micrometers cannot be used to form relatively thin material. Unless relatively thick material is desired, the fibers should not exceed those dimensions. Also, where the aspect ratio (ratio of length to diameter) is lower than about 100, there is insufficient crossing and interlacing among the fibers to provide the requisite strength in the resulting material. Accordingly, fibers with aspect ratios of at least about 100 are used. If the fibers have diameters less than about 5 micrometers, they (1) tend to break up during dispersion, thereby decreasing their aspect ratio and yielding a material having reduced void volume as well as reduced strength, and (2) tend to have high resistance to flow of liquids. PTFE fibers of the desired type are commercially available, e.g., from E. I. DuPont de Nemours and Company, Inc.
In Hurley et al., the fluorocarbon polymer binders include fluorinated ethylene-propylene copolymers, and particularly copolymers of tetrafluoroethylene (TFE) and hexafluoropropylene referred to as FEP. Other fluorocarbon binders such as perfluoroalkoxy polymer ("PFA") may be used so long as they exhibit the desired resistance to high temperatures and caustic chemical environments and provide the requisite bonding of the PTFE fibers upon curing.
An emulsion/suspension incorporating the binder is used. The concentration of the fluorocarbon binder (weight percent solids) in the emulsion as supplied is typically in the range of from about 45 to to about 65 weight percent. The particles of the fluorocarbon polymer binder in the emulsion generally have sizes in the range of from about 0.05 to about 1.0 micrometer, preferably 0.1 to about 0.4 micrometer. The dispersions are generally aqueous and typically contain from 3 to 12%, more typically 5 to 7%, by weight of volatile nonionic and anionic wetting agents. The dispersions are generally diluted with water before use, typically to 0.5 to 25% by weight or higher, more preferably from about 5% to about 15% by weight, of the fluorocarbon binder.
Appropriate wetting agents include ethoxylated alkyl phenol, alkaryl polyether alcohol (Triton X100, available from Rohm & Haas Company).
The fluorocarbon binder is required to set or cure before any substantial shrinkage or melting of the PTFE fibers occurs. For this reason, thermoplastic fluorocarbon binders having a proper melting point range, such as FEP, are preferred. A particularly preferred FEP fluorocarbon polymer binder is FEP 120 available from E. I, DuPont de Nemours and Company, Inc. In dispersion form, a thermoplastic resin like FEP is set or cured by flowing the resin over the fibers to coat them and heating the dispersion to cause the fibers to adhere to one another at crossover points.
As noted in Hurley et al, because of carbonaceous residues remaining from their manufacture, staple PTFE fibers are typically a rich, chocolate brown color. Aesthetically, this is undesirable in filters, filter support and drainage structures. If not removed, the carbonaceous residues remain in the filter structure as extractables, i.e., materials which can leach from the filter structure during service.
Accordingly, Hurley et al. recite alternative fiber bleaching/whitening methods practiced before the fibers are subject to laydown. The first is by chemical means, e.g., by heating the fibers in concentrated sulfuric acid heated to approximately 600.degree. F. to which is added slowly concentrated nitric acid until the fibers turn white. The second, preferred, method is thermal bleaching of the substrate fibers before laydown. Thermal bleaching of the fibers is accomplished, e.g., by gradually increasing the air temperature over an extended period of time from about 450.degree. F. up to about 570.degree. F. and maintaining such temperature for several hours until the fibers are substantially free of carbonaceous material (whitened). Hurley et al point out that thermal bleaching will typically result in a shortening of the fibers and an increase in their diameter. Depending on the desired size of the fibers in the formed structure, the fibers can be chopped to the desired length before or after their thermal bleaching.
The bleached fibers are used in a stabilized dispersion used to prepare the porous structures. The dispersion is comprised of a liquid carrier medium, the bleached PTFE fibers, and a stabilizing agent. A single constituent may serve to both stabilize the dispersion of PTFE fibers and, upon drying, to bind the fibers to each other, thereby providing green strength. The stabilizing agent is mixed with the liquid carrier medium, preferably water, in an amount such as to provide the requisite concentration and until a uniform dispersion is obtained. Then the bleached PTFE fibers, in appropriate amount, are then added and mixed to provide a uniform dispersion or suspension of the fibers in the liquid medium. The fiber containing dispersion is stabilized, i.e., the PTFE fibers remain in suspension and will not settle out at a rate fast enough to adversely affect the laydown of the dispersion.
Hurley et al also describe the use of a stabilizing agent to increase dispersion viscosity. For example, with Carbopol 941 (a polyacrylic acid available from B. F. Goodrich Chemicals Company), the addition of a neutralizing base, ammonium hydroxide, serves to neutralize the polyacrylic acid and increase the viscosity substantially, e.g., to about 800 to 20,000 centipoise. Such systems are very thixotropic. When vigorously agitated, they have a low effective viscosity and, hence, are very effective in dispersing the PTFE fibers. Since these dispersions are very stable, they may be prepared in advance of the time they are used without settling out of the fibers.
Carbopol 941 may be used without the addition of a neutralizing base, albeit addition of a neutralizing base is preferred. When a neutralizing base is not used, more of the Carbopol must be added to achieve the desired viscosity. Viscosities in the range of from about 800 to about 20,000, preferably in the range of 1,200 to 6,000, centipoise may be used.
Also, a fugitive binder may be introduced into the fiber dispersion to give the laydown sufficient strength to withstand handling until curing of the fluorocarbon binder is effected. Both the stabilizing agent and the fugitive binder, if used, preferably are fugitive but may require separate treatment for removal. Preferably Carbopol, particularly Carbopol 934, Carbopol 940, and Carbopol 941 is used since it provides a single component which acts as both a fugitive stabilizer and fugitive binder.
Hurley et al. describe that the amount of PTFE fibers present in the stabilized dispersion should be such that a uniform and complete dispersion is achieved with the minimum amount of water. Generally from about 0.5 to about 20, and preferably from about 1 to about 5, grams of fibers per liter of the dispersion comprising the stabilizing agent, liquid carrier, and fiber mixture is satisfactory. Lower amounts can be used, but are less economical. Higher amounts are more difficult to disperse and may lead to clumping or aggregation of the fibers and a non-uniform laydown.
The amount of stabilizing agent/binder varies according to the particular agent chosen, but in every case it should be sufficient to provide a stabilized dispersion as described above. Typically, mixtures of the liquid medium and stabilizing agent have viscosities of from about 800 to about 20,000, and preferably from about 1,200 to about 6,000, centipoise, prior to incorporation of the fibers. Generally, from about 0.4 to about 4 grams per liter of the stabilizing agent/binder and, preferably, from about 1 to about 2 grams per liter of the liquid carrier and stabilizing agent/binder mixture are sufficient.
The dispersion is prepared with conventional equipment, e.g., a baffled Cowles mixer or beater. Generally, beating at a tip speed of from about 3,000 to about 4,500 feet per minute for from about 30 to about 120 minutes is sufficient to produce a uniform, complete dispersion of the fibers. Excessive tip speed and mixing times are not desirable due to a loss in viscosity caused by shearing of the molecules of the stabilizing agent. The surface tension of solutions of the desired characteristics which have been thickened with Carbopol is above 70 dynes per centimeter and was measured as 74.2 dynes per centimeter for a 1.4 grams per liter of Carbopol 941 mixture.
Hurley et al. describe that the stabilized dispersion may be laid down on any suitable porous support, e.g., a woven glass cloth or wire mesh. The laydown should be of uniform thickness and spreading means may be employed for that purpose. Excess liquid carrier then is drained from the laydown, preferably by drawing a vacuum through the support. Conventional equipment, such as a Fourdrinier, may be used. To avoid premature and uneven drainage of the laydown, an impervious sheet, such as a plastic sheet, may be placed over the porous support prior to laydown of the dispersion. After distribution of the dispersion over the support, the plastic sheet is removed and the liquid is drained.
The next step discussed in Hurley et al. is drying the laydown before application of the fluorocarbon polymer binder. Drying is accomplished by drawing ambient or warmed air through the laydown, by infrared radiation or with conventional oven equipment. With an infrared heater, 6 to 9 minutes is satisfactory when operated at about 6 watts per square inch of the fibrous structure laydown at a distance of 3 to 4 inches.
In Hurley et al., the fluorocarbon binder emulsion is applied to the laydown by spraying, dipping, or other conventional techniques. The amount of fluorocarbon binder emulsion applied varies according to the concentration of the fluorocarbon binder in the emulsion. The concentration of the emulsion and the amount of emulsion applied to the laydown, whether in a single application or in multiple applications, must be such that sufficient amounts of fluorocarbon binder are provided to ensure adequate bonding of fiber-to-fiber contacts to impart structural integrity to the resulting fibrous structure. Also since wicking can occur, it may be desirable to undertake multiple binder emulsion steps.
As delineated in Hurley et al., the amount of fluorocarbon binder distributed in the final product, i.e., the porous fibrous structure, generally ranges from about 5 to about 45, preferably from about 10 to about 35, weight percent based on the weight of fibers. Amounts below 5 weight percent may be used, however, when less binding is required. Undesirable webbing, the formation of polymer films, generally results from amounts greater than 45 weight percent.
The binder impregnated laydown is again dried after binder application. The fluorocarbon impregnated fluorocarbon fiber structure is cured by heating at a higher temperature than is required for drying. For thermoplastic fluorocarbon binders the curing temperature and time should be such that the fluorocarbon binder is allowed to melt and flow. For example, when FEP is used, setting or curing will be carried out typically at from about 515.degree. F. to about 650.degree. F. for from about 20 seconds to about 1 minute.
Finally, Hurley et al. discuss the problems of introduction of extractables into the filter media by the various laydown constituents. To the extent that extractables are introduced by the stabilizing agent, and fugitive binder, if used, any residual surfactant from the fluorocarbon dispersion, etc., Hurley et al, rely on an acid extractables reduction step to reduce the level of extractables. The acid extractables reduction step involves exposure of the cured laydown to heated mineral acid, such as 70% reagent grade nitric acid at 230. to 250 F for five hours followed by rinsing with pure water and drying of the structure.
While nitric acid is an effective way to whiten the medium it is not without significant drawbacks. For example, three basic difficulties have been observed. First, the white color is reversible in subsequent manufacturing steps and in filter service. The medium discolors when heated to temperatures in excess of approximately 250.degree. F. This tendency to discolor causes manufacturing problems as, for example, in side sealing and end capping operations. The color depends upon the maximum processing temperature. After placement into service where the service temperatures exceed 250.degree. F. the filter medium discolors. The color that develops varies from tan through various shades of brown to a purplish hue. The reappearance of color in the medium is indicative of the presence of organic compounds and/or decomposition products which remain in the medium and which have the potential to be extracted in service.
Secondly, reliance on repeated rinsing that removes most but not substantially all of the nitrate ions is generally unsatisfactory under the increasingly stringent standards of industry. Quality specifications now often require that the level of total extractables, especially for nitrate ions, be reduced dramatically, e.g. even below 53 ppb/ft.sup.2. Further sequential rinsing of the medium to lower extractable nitrate levels introduces other problems. Since the medium is hygroscopic, prone to dewet, after rinsing and drying, it must be rewet with a low surface tension organic liquid such as isopropyl alcohol- often at the start of each step which involves additional labor. Moreover, the volume of deionized water contaminated with nitrate and organic wastes, alcohol, introduce increased waste disposal concerns and must be properly discarded or reprocessed.
Thirdly, residual nitrates not only contribute to filter medium discoloration but also have the potential to contaminate the filtrate when place in service. Even the most thorough rinsing of the medium does not eliminate the residual nitrate levels to below the range of several hundred to a few thousand ppb/ft.sup.2.
Neither the PTFE (fibers) nor the fluorocarbon binder, themselves, are discoloring. It is, in fact, the processing aids that cause the discoloration/contamination problems. The wetting agent in the binder and to a lesser degree the dispersant and stabilizer/fugitive binder have been found to be the sources of discoloration. These processing aids are necessary but should be removed entirely once they have served their purposes. However, due to fusion of the binder with heat, these processing aids and residues therefrom are entrained in the fused fluorocarbon of the laiddown medium. These undesirable, entrained constituents must be removed to prevent discoloration of the filter medium and to prevent potential contamination in service.
As a substitute for the above-described nitric acid bleaching step, bleaching with hot Hydrogen Peroxide (30%) has been attempted. Unfortunately, it is not as effective a bleaching agent as nitric acid. Like nitric acid, it poses safety and disposal issues. Hot hydrogen peroxide can produce uncontrolled decomposition reactions which are also potential sources of contamination. Color returns upon moderate heating.
Conventionally, heat treatment after laydown was shunned because of the prospect of fiber shrinkage, breaking the fiber cross-over bonds created with the fluorocarbon binder, and reducing the strength and structural integrity of the filter media. The prior art specifically avoided prolonged exposure of fluorocarbon fiber based media to high temperatures since it was believed that after laydown, the fiber shrinkage adversely impacted on filter integrity.
Convention also dictated that protracted exposure to high temperatures would lead to extensive shrinkage of the medium and that the thickness of the medium would be unacceptably decreased. Given the prerequisite that the medium possess a high void volume, the prospect of a substantial reduction of medium volume deterred high temperature treatment. Furthermore, and particularly applicable to fluorocarbon media, there was a concern that protracted exposure to high temperatures would lead to unacceptably high fluoride extractable levels.
Finally, since media bleached with nitric acid and hydrogen peroxide discolored when heated to high temperatures, it did not appear reasonable to employ high temperature exposure to bleach the medium.